Hamilton Oh wrote:How is the -CH3 group in acetic acid less electron withdrawing that the -CCl3 group in trichloroacetic acid?what does electron withdrawing mean?

And how is formic acid a stronger acid that trichloroacetic acid?

Cl is more electronegative than Hydrogen; thus, when chlorine is attached to the central atom of carbon, it will more electrons will be drawn toward the chlorine to help stabilize the structure. Increased stability increases acidity. Electron withdrawing means that more electrons are going to be drawn away from the central atom and toward the more electronegative element. Formic acid is not a stronger acid than trichloroacetic acid because of its extra Hydrogen atom attached, this makes formic acid less electron withdrawing and more electron donating due to two positively charged hydrogen atoms.